1. Field of the Invention
This invention relates to a process for the solvent extraction of tar sands. More particularly, a low-boiling hydrocarbon solvent is used in a solvent extraction process in which bitumen and inorganic fines are separated from the extracted sands at a low solvent: bitumen ratio in an extraction zone containing a classifier and countercurrent column and solvent is recovered from the extracted tar sands in successive fluid-bed drying zones.
2. Description of the Prior Art
Among the many approaches considered for separating the petroleum fraction from tar sands, the hot water process represents a well-developed recovery technique. The principal disadvantage of this process is the enormous volume of aqueous tailings. These tailings contain a stable suspension of inorganic fines. Since no economically viable schemes have been devised for removing these suspended fines thus permitting water recycle, the tailings are held in sludge ponds which are both a major expense and an environmental hazard.
Various solvent extraction schemes have been proposed as alternatives to aqueous processes and are attractive because of the ease of separation of tar from sand. On the other hand, such a process must provide for a simple and nearly complete solvent recovery since the solvents are much more expensive than tar sands (present value of the latter is about 0.3.cent./lb versus about 16.cent./lb for typical solvent).
A solvent extraction process employing aromatic solvents boiling in the range of 82.degree. to 138.degree. C. is disclosed in U.S. Pat. No. 4,139,450 (Hanson et al.). The proces employs a settler followed by a countercurrent extractor but is directed to tar sands other than Athabasca tar sands. Water is removed initially by hot solvent prior to the extraction process. The settler has an overflow of mostly inorganic fines and bitumen and an underflow of coarse solids.
U.S. Pat. No. 4,071,433 (Hanson) describes a liquid slurry process for extracting tar sands in which the tar sands are slurried with an oil and divided in a centrifugal zone into streams containing coarse and fine sands. The fine sand stream is fed to a coker where the fines act as nuclei in coke formation. The coarse sand stream is filtered by means of a hot oil filter and subsequently dried. Similarly, U.S. Pat. No. 4,071,434 (Gifford,II) discloses a process for removing sand from an oil- or water-tar sand slurry wherein the slurry is passed through a centrifugal classifier and divided into fine and coarse sand streams. The coarse sand stream is then subjected to a particle removal step and then fed, together with the fine sand stream, to a coking zone.
The use of a C.sub.5 to C.sub.9 paraffinic hydrocarbon solvent in a solvent:bitumen ratio of 2 to 1 to 10 to 1 for a countercurrent extraction of tar sands is disclosed in U.S. Pat. No. 4,036,732 (Irani et al.). In this process, the tar sand is first passed to a slurry zone where it is mixed with water containing a small amount of phenol and then to an extraction zone in which a three-phase system must exist. U.S. Pat. No. 3,475,318 (Gable et al.) discloses a process for selectively removing oil from tar sands whereiin a C.sub.5 to C.sub.9 saturated hydrocarbon or a mixture thereof with a C.sub.5 to C.sub.9 aromatic hydrocarbon is employed to extract a bed of tar sand deposited on a moving filter. A solvent ratio between 2:1 and 10:1 can be employed.
A solvent extraction process employing a simple countercurrent column is described in U.S. Pat. No. 3,553,099 (Savage et al.). A flowable slurry of tar sand is extracted in a bed by an upwardly flowing toluene wherein fresh toluene is added at 0.25 gal/lb of tar.
U.S. Pat. No. 3,118,741 (Faulk) discloses a process for recovering hydrocarbon solvent from sands such as that obtained from the solvent extraction of bituminous sands. Hot sand is passed through a sloping contact zone together with a gaseous material and then into a separator where gaseous material is withdrawn through the upper portion whereas the solids in the lower portion are maintained in a fluidized condition. Countercurrent extraction of sand with a liquified normally gaseous hydrocarbon and subsequent removal of the hydrocarbon from sand at reduced pressure in a flash drum is described in U.S. Pat. No. 3,131,141 (West).
It would be highly desirable to have a solvent extraction process which would be economically competitive with the hot water process, i.e., provide very high solvent recovery rates, while at the same time eliminating environmental hazard posed by the extensive aqueous tailings produced by the hot water process. The solvent recovery system must have minimal energy and capital requirements and must also avoid emulsion formation which can occur in a system containing surface active solids, water and solvent.